Basal plane III-nitride laser diodes have been commercialized for wide ranging technologies, including pico projectors for solid state RGB displays, optical data storage and automotive headlights. Active research is focused in solid state lighting and visible light communications. Despite widespread commercialization, c-plane devices are affected by the inherent spontaneous and piezoelectric polarizations of the basal plane. These polarization effects are significantly reduced in semipolar planes of GaN, creating a vast design space for III-nitride optoelectronic devices.

III-nitride semipolar laser design and performance are considerably affected by the material composition and growth conditions of the cladding layers. The bottom cladding design is limited by stress relaxation of ternary alloys while the top cladding is limited by the growth time and temperature of the p-type (Al,In,Ga)N layers. These design limitations have been independently addressed by using limited area epitaxy (LAE) to enable n-AlGaN bottom cladding layers and by using thin p-GaN and transparent conducting oxide (TCO) top cladding layers.

In this work, we investigate a new laser design that simultaneously incorporates LAE-enabled n-AlGaN bottom cladding and thin p-GaN and TCO top cladding layers in (2021) III-nitride laser structures. We evaluate the performance of two different TCOs as the top cladding layer: indium-tin-oxide (ITO) and zinc oxide (ZnO). Thorough optical modeling of the LAE-TCO laser design will be discussed for various Al compositions in the LAE-enabled n-AlGaN bottom cladding and varying p-GaN thicknesses in the top cladding. The LAE-TCO laser design was first demonstrated using ITO as the top cladding layer, with pulsed lasing achieved at 446 nm with a threshold current density of 8.5 kA/cm2 and a threshold voltage of 8.4 V. Insights from the optical modeling in conjunction with improvements in the LAE-TCO laser fabrication process led to the demonstration of the LAE-TCO design using ZnO as the top cladding layer, with pulsed lasing achieved at 445 nm with a threshold current density of 5.6 kA/cm2 and a threshold voltage of 6.7 V. This notable improvement in threshold current density and voltage led to the first continuous wave (CW) operation of blue (2021) LAE-ZnO laser structures.